A spectrograph is a device for measuring the intensity of light or other electromagnetic radiation as a function of wavelength (e.g., where the wavelength falls within the visible spectrum, a color wavelength). Spectrographs may be used in an enormous variety of applications, including for example the determination of physical attributes of materials, substances, or objects by analyzing spectral characteristics of light emitted by, reflected from, or transmitted through such materials, substances, or objects. At a fundamental level, a spectrograph can consist of an input aperture through which light is sent into the device, a dispersing element which bends the light beam through different angles depending on wavelength, and a camera subsystem which translates those angles into positions along a detector.
Among fundamental performance parameters of a spectrograph are (1) its bandpass, i.e., the range of wavelengths which it can detect, (2) its spectral resolution, i.e., the size of the smallest spectral feature or detail which can be discerned, and (3) its throughput, i.e., the percentage of light entering the device which is ultimately measured by the spectrograph sensor.
For many applications, bandpass, spectral resolution, and device throughput are all important. Higher spectral resolution can give a sharper view of the target spectrum, permitting a more detailed analysis, and higher throughput can provide a stronger measurable signal, improving the quality of the data (often quantified as the “signal-to-noise ratio”).
Many, if not most, current conventional spectrographs represent tradeoffs between these parameters. For example, the spectral resolution of a particular spectroscopic device is often limited by the size of the image of the light input aperture as measured along the direction of dispersion. To achieve higher resolution, many spectrographs employ a “slit,” such as in the form of a rectangular aperture which is narrow along a dispersion direction and relatively taller in a perpendicular dimension. A narrow slit can yield a narrow monochromatic image on the detector, and thus a sharper view of spectral features. However, a narrow slit can also block a large fraction of the input light if, for example, the initial light source is larger than the slit width. Such a spectrograph therefore typically sacrifices throughput in order to achieve higher resolution, with resultant reduced data quality. Analogous difficulties may arise where, for example, alternate light sources such as fiber optic cable or other inputs are employed.